Drum-type brakes as are commonly used for motor vehicle wheel-braking systems and in many other applications, frequently known also as internal, expansion brakes, customarily make use of brakeshoes whose friction surfaces are formed by curved brake linings made by compression, generally under both heat and pressure, of a composite material including a thermally activatable binder.
The brake linings generally are circular-cylindrical segments and may be bonded, upon pressing, to a support to form the brakeshoe or may be otherwise secured to the metal support in the production of the brakeshoe.
In the conventional method of making such curved brake linings, a press ram is displaced downwardly toward a mold or die for shaping the material to be formed into the brake lining in the press.
The bottom of the mold or die is formed with a curved floor having the configuration of a circular cylindrical segment and rising along the edges of this wall are opposite bars of side walls lying in mutually parallel vertical planes.
In this curved-bottom mold or form, the composition which is to be bonded under heat and pressure to form the brake lining is introduced.
In the past, this composition has frequently been a fibrous mixture and is metered into the mold or form to a height sufficient to enable the desired thickness of the brake lining to be pressed by the press ram. The pressing may be carried cut in one or more pressing stages and the end product is generally desired to have a predetermined and usually constant thickness and the desired density of the material. In practice this means that the end regions of the mold or form must be filled to a greater height with the composition than the region of the center line or axis.
Increasing demands for higher quality brake linings in recent years has led to the desire to use finely divided mixtures which have a granular consistency as distinct from the fiber structure of earlier compositions.
Such mixtures tend to be more flowable than the earlier compositions and it has been found that these mixtures are not readily able to retain the predetermined contour on a curved bottom. Furthermore, there are unavoidable vibrations in the operation of the system which cause flow of the material in the mold and often preclude the formation of uniform thickness, and not producing braking linings of uniform density as is essential for optimum braking results.
This is especially the case when the contour of the pile of the brake-lining composition in the mold is to conform to a parabola and is formed by a pattern, as is often the case.